Because of the waste of fuel resulting from the use of burners whose main burner ignition is accomplished by a standing pilot burner, the fuel burner control industry is shifting to electrical igniters of various types to ignite the fuel each time heat is demanded. The great advantage of the standing pilot technology is that of very high reliability and safety. The presence of a standing pilot flame can be easily and cheaply sensed by the heat it generates, and the pilot flame heat sensor can then override opening of the main valve if the pilot flame is not sensed. It goes without saying that it is important that the main valve not be opened if there is no means to ignite the fuel once it starts flowing.
Of the various types of electrical igniters, one which has reliability and long life is the hot surface igniter, and for these reasons has become very popular. A hot surface igniter typically comprises a small element formed of silicon carbide or other conductive refractory material. Current is passed through this element until its temperature reaches upwards of 1000.degree. F., which is sufficient to ignite a combustible mixture of air and some fuel such as natural gas or propane fuel.
It is not so simple to directly detect proper operation or functionality of a hot surface igniter as it is for a pilot flame, because the total amount of heat which such devices emit is substantially smaller than that released by a pilot flame. Direct detection of hot surface igniter operation is exemplified by U.S. Pat. Nos. 4,323,342 (Sommers et al.) and 3,676,042 (Osborne). It is also possible to sense the presence of the flame which the ignition operation is intended to produce, and if flame is not present within a short time after opening the valve, then close the valve. One can find this design in U.S. Pat. Nos. 4,190,414 (Elmy); 4,360,338 (Katchka); 4,269,589 (Matthews); and 4,806,095 (Goldstein et al.) The disadvantage of this latter approach is that malfunction is sensed only after fuel has flowed for a time without ignition. Furthermore, if more than a small amount of fuel has flowed without being ignited, it is not safe to retry ignition until the burner enclosure has been purged, particularly if the main burner is being directly ignited. Such purging requires a blower, which smaller burner systems typically do not have. Therefore it is necessary to disable the burner control until serviced, which is inconvenient, particularly if the fault was only an intermittent one.
Another factor which must be considered involves the operation of the thermostat, the closing of whose contacts typically generates the demand signal calling for heat. Loads applied to these thermostats should in general be within the range of current required to operate a valve actuator. If the load is much larger, life of the thermostat contacts is shortened. If the load is much smaller, then the heat anticipation required for proper thermostat response is adversely affected.
Accordingly, efficiency, safety, and convenience of such burner systems employing igniters to establish the flame can be improved if another way of establishing proper operation of the igniter before opening the fuel valve can be found.
My copending application entitled "Fuel Burner Having an Intermittent Pilot With Pre-Ignition Testing" and having the same assignee as this application tests the operation of an igniter such as a hot surface igniter by sensing the current which the igniter draws and the voltage across it. If both are within preselected limits established for the igniter, proper igniter operation can be assumed.